Thermal printheads have been widely used for a printer of an office automation apparatus such as a facsimile machine, a printer of a ticket vending machine and a label printer. As is commonly known, a thermal printhead selectively provides heat to a printing medium such as thermosensitive paper or thermal-transfer ink ribbon to form needed image information.
Thermal printheads are divided mainly into thin film-type thermal printheads and thick film-type thermal printheads, depending upon methods of forming their heating resistors (heating dots) and electrode conductor layers for example. In a thin film-type thermal printhead, a heating resistor and an electrode conductor layer are made in the form of a thin film on a substrate or a glass glaze layer by sputtering for example. On the other hand, in a thick film-type thermal printhead, at least the heating resistor is made in the form of a thick film through such steps as screen printing and sintering.
In general, for thermal printheads, a series of linear heating dots are formed preferably adjacent to a longitudinal edge of the head substrate. This is because the arrangement of disposing the series of heating dots adjacent to a longitudinal edge of the head substrate advantageously makes it possible to avoid interference with the printing medium as well as to increase degrees of positioning freedom and improve printing quality by holding the head substrate relative to the platen at a certain angle.
However, when the series of heating dots are adjacent to a longitudinal edge of the head substrate, space for formation of the common electrode pattern is correspondingly reduced, thereby failing to provide a sufficient current capacity (current passage) necessary for heat generation. As a result, the resistance of the common electrode pattern will cause a problem of irregular heat generation at the heating dots due to a voltage drop along the series of heating dots, which results in deterioration of printing quality. Particularly for color printing, which has been coming into wider use recently, it is highly required to ensure a large current capacity since all of the heating dots are frequently heated simultaneously to perform so-called "solid printing."
To meet such a requirement, International Publication WO 95/32867 discloses a thermal printhead with the arrangement shown in FIGS. 5 and 6 of the attached drawings of the present application. (Note that the above international publication was published on Dec. 7, 1995, which is later than the priority date of the present application, Jun. 13, 1995, so that the international publication is not to be regarded as prior art against the present application.) The above-mentioned thermal printhead will be described below.
The thermal printhead illustrated in FIGS. 5 and 6 includes a head substrate 11 of an insulating material such as alumina-ceramic for example. The head substrate 11, which is rectangular in cross section, includes an obverse surface 11a, a reverse surface 11b opposite to the obverse surface 11a, a first longitudinal edge surface 11c and a second longitudinal edge surface 11d opposite to the first longitudinal edge surface 11c. The obverse surface 11a of the head substrate 11 is formed with a glass glaze layer 12 as a heat reservoir. The glass glaze layer 12 includes a convex portion 12a, adjacent to the first longitudinal edge surface 11c of the head substrate 11, which has a curved cross section.
The surface of the glaze layer 12 is formed with a thin film of a resistor layer 13. The resistor layer 13 is divided by slits S (see FIG. 6) at a predetermined pitch to extend transversely of the head substrate 11 (that is, perpendicularly of the longitudinal edge surfaces 11c, 11d of the head substrate 11).
The surface of the resistor layer 13 is formed with a common electrode pattern 14 adjacent to the first longitudinal edge surface 11c of the head substrate 11, and individual electrodes 15 which are spaced from the common electrode pattern 14 and extend from the convex portion 12a of the glaze layer 12 toward the second longitudinal edge surface 11d of the head substrate 11.
The slits S extending to the common electrode pattern 14 electrically insulate the individual electrodes 15 from each other.
As described above, the individual electrodes 15 are spaced from the common electrode pattern 14. Thus, the resistor layer 13 is exposed between the common electrode pattern 14 and the individual electrodes 15, and the exposed portions function as heating dots (heating regions) 13a linearly extending along the first longitudinal edge surface 11c of the head substrate 11.
The heating regions (heating dots) 13a of the resistor layer 13, the common electrode pattern 14 and the individual electrodes 15 are covered with a protecting layer 20. Due to the protecting layer 20, the common electrode pattern 14 and the individual electrodes 15 are prevented from getting oxidized through contact with the air and worn out through contact with a printing medium (not shown).
The common electrode pattern 14 is electrically connected, at an end closer to the first longitudinal edge surface 11c of the head substrate 11, to an auxiliary electrode layer 16 made of a metal such as aluminum for example. Thus, every portion of the common electrode pattern 14 is electrically connected to each other via the auxiliary electrode layer 16, thereby being kept at a same electrical potential. In other words, the auxiliary electrode layer 16 functions as a member commonly connecting all portions of the common electrode pattern 14.
The auxiliary electrode layer 16 covers the first longitudinal edge surface 11c of the head substrate 11, the reverse surface 11b and the second longitudinal edge surface 11d. Thus, the auxiliary electrode layer 16 has a large area to allow increased electrical passage so that the voltage drop which might otherwise be caused longitudinally of the thermal printhead is substantially eliminated. As a result, a large amount of current is provided even for an instance where all heating dots 13a are simultaneously heated (that is, even for solid printing), thereby preventing deterioration of printing quality.
The thermal printhead with the above arrangement may be formed by a method illustrated by FIGS. 7a-7j for example.
First, as shown in FIG. 7a, an alumina-ceramic master substrate 11' dimensionally corresponding to a plurality of head substrates is prepared. The master substrate 11' will be divided later along longitudinal division lines DL1 and transverse division lines DL2 to provide the plurality of head substrates.
Then, as shown in FIG. 7b, a master glaze layer 121 is formed by sintering a glass paste which is applied over the master substrate 11'.
Then, as shown in FIG. 7c, a groove 17 extending into the thickness of the master substrate 11' is formed by using a dicing cutter (not shown) which cuts through the master glaze layer 12' along a predetermined longitudinal division line DL1. Thus, the master glaze layer 12' is divided into separate glaze layers 12.
Then, as shown in FIG. 7d, the glaze layer 12 is formed with a convex portion 12a adjacent to the groove 17 by heating the master substrate 11' at a temperature of about 850.degree. C. for about 20 minutes. The formation of the convex portion 12a is realized under the influence of the surface tension of the glass material which is in a liquidized state by the heating.
Then, as shown in FIG. 7e, a resistor layer 13 including tantalum nitride as the main component is made in the form of a thin film over the glaze layer 12 by reactive sputtering.
Then, as shown in FIG. 7f, a conductive layer 18 of e.g. aluminum is formed on the resistor layer 13 by sputtering.
Then, as shown in FIG. 7g, after forming the slits S (see FIG. 6) by etching the resistor layer 13 and the conductive layer 18, only the conductor layer 18 is partially removed by etching for exposure of portions of the resistor layer 13 to form heating dots 13a. Thus, the conductor layer 18 is divided into the common electrode pattern 14 and the individual electrodes 15.
Then, as shown in FIG. 7h, the master substrate 11' is divided by a dicing cutter (not shown) along the respective division lines DL1, DL2 to provide separate head substrates 11.
Then, as shown in FIG. 7i, while each head substrate 11 is being moved in the direction of arrow X, conductive metal is sputtered from below to be fixed on the first longitudinal edge surface 11c, the reverse surface 11b and the second longitudinal edge surface 11d for forming a proper thickness of the auxiliary electrode layer 16 made of aluminum for example.
Finally, as shown in FIG. 7j, a protecting layer 20 is formed to cover the common electrode pattern 14, the individual electrodes 15 and the heating dots 13a or the exposed regions of the resistor layer 13.
In the method described above, the formation of the auxiliary electrode layer 16 is performed after the division of the master substrate 11' into the separate head substrates 11 (see FIGS. 7h and 7i). However, it has been found that the following problems will occur by the method of making the auxiliary electrode layer 16 described above.
First, since the auxiliary electrode layers 16 are formed after the master substrate 11' is divided into the plurality of separate head substrates 11, specially adjusted magazines and tools are needed to deal with the plurality of head substrates 11 individually, which results in a larger equipment cost. Further, the process of making an auxiliary electrode layer 16 individually for each of a plurality of head substrates 11 will make the production rate low. Such a factor, together with the increased equipment cost, will increase the production cost.
Secondly, when the auxiliary electrode layer 16 is formed for each separate head substrate 11, the conductive metal to be sputtered can easily reach the obverse surface of the head substrate 11, going beyond the common electrode pattern, and may further extend to the heating dots 13a or the exposed portions of the resistor layer 13. As a result, the auxiliary electrode layer 16 partially or wholly covers the heating dots 13a to prevent the heat generation at the heating dots 13a.
Thirdly, when the auxiliary electrode layers 16 are formed after dividing the master substrate 11' into the plurality of separate head substrates 11, apparatus for carrying and supporting the separate head substrates 11 will come into direct contact with the head substrates 11, thereby possibly causing a secondary damage to the obtained thermal printheads. On the other hand, when the master substrate 11' is not divided, the carrying and supporting of the master substrate 11' can be performed with the use of the marginal portions thereof. Thus, there are much less possibilities of causing a damage to the head substrates 11 which will be separated afterward.